Current commercial practice utilizes aqueous flushing solutions that contain amines and an organic solvent selected from lower alcohols and glycol ethers. The flushing solutions can be used to remove paint and other coatings from surfaces and coating application equipment. Generally, alkanolamines like dimethylethanolamine, isopropanolamine and diethanolamine are used. Flushing solutions can be prepared from flushing concentrates, which are marketed to the end user. The flushing concentrates are usually diluted to 5% to 15% by volume with deionized water.
U.S. Pat. No. 5,632,822 discloses a water in oil emulsion paint remover used to strip cured paint from surfaces. This emulsion is prepared by adding a non-polar mixture containing benzyl alcohol to a polar mixture containing water, an alkanolamine and polyethoxylated sorbitan monooleate. U.S. Pat. No. 6,303,552 discloses a paint stripper composition containing 20% to 50% benzyl alcohol and 0.5% to 5% by weight of an accelerator. The accelerators include alkanolamines including ethanolamine, triethanolamine, or any combination thereof. This paint stripper can also contain an emulsifier such as polyethoxylated sorbitan monolaurate, polyethoxylated sorbitan monopalmitate, polyethoxylated monostearate, polyethoxylated tristearate, polyethoxylated monoleate, trioleate polysorbates, and mixtures thereof. Chelating agents such as phosphoric acid, citric acid salts and EDTA can also be included in the stripper composition.
The pH of flushing solutions usually ranges from 8.5 to 11. The alkaline pH causes surface corrosion of the coating application equipment, e.g., the aluminum or aluminum alloy spray equipment used in the auto paint industry. The spray equipment typically uses aluminum or aluminum alloy rotating bell cups to atomize the paint. The aluminum bell cups have serrated edges around the outer perimeter. As the paint flows past the serrated edges, the paint is atomized. Corrosion of the rotating bell cups over time, particularly the serrated edges, affects the atomization of paint particles. As the atomization properties of the equipment changes, the quality of the paint finish also changes. As a result, corrosion of the spray equipment leads to a problem of quality control.
The addition of silicates and other corrosion inhibitors to the flushing solutions can be used to reduce the corrosion of aluminum and aluminum alloys. However, in the case of silicates, sufficient water must be present in the concentrate to maintain the silicates in solution. For example, it is very difficult to prepare stable silicate-containing flushing concentrates with less than 50% by weight water because the silicates will precipitate from the concentrate over time. The resulting flushing concentrate no longer possesses the chemical inhibition properties it was designed for. As a result, there is a need to develop flushing solutions with corrosion inhibitor systems that are stable when the flushing solution is in concentrate form.